metamorphic
Banded gneiss pebble
Gneiss, chiefly quartz–feldspar–mica with dark biotite/amphibole bands
AI-generated identification · may be incorrectTypically 6–7 Mohs; gray, tan, and black compositional banding, with a granular texture and dull to slightly glassy luster. The rounded shape is from water abrasion; individual mineral cleavage is visible rather than a uniform rock cleavage.
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Physical properties
Typically 6–7 Mohs; gray, tan, and black compositional banding, with a granular texture and dull to slightly glassy luster. The rounded shape is from water abrasion; individual mineral cleavage is visible rather than a uniform rock cleavage.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, causing minerals to recrystallize and segregate into bands, commonly during Precambrian–Paleozoic mountain building. The specimen was later eroded into a stream or beach pebble.
Uses & applications
Used as aggregate, dimension stone, landscaping rock, and decorative polished stone; this small specimen has modest collector value.
Geological facts
The alternating light and dark layers are compositional foliation, not sedimentary bedding, and can record intense continental collision and deep-crustal pressure.
Field identification & locations
Identify it by persistent light quartz/feldspar bands alternating with dark mica or amphibole, granular crystals, and streaked foliation; common in glaciated terrains, mountain belts, river gravels, and beaches. Value is usually $2–$15 for a common pebble, or $20–$75 if unusually patterned and polished.
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